The present invention relates to a solid electrolytic capacitor comprising a solid electrolytic layer of a conducting polymer formed on a dielectric film, and in particular, it relates to improvement of the solid electrolytic layer for the purpose of providing a solid electrolytic capacitor having large capacitance and exhibiting low impedance in a high frequency region.
In accordance with recent trend in reducing the size and weight of electronic equipment, there are increasing demands for a compact capacitor having large capacitance and exhibiting low impedance in a high frequency region.
Capacitors conventionally used in a high frequency region are a plastic capacitor, a mica capacitor and a laminated ceramic capacitor, all of which are large in size and have a capacitance difficult to increase.
As a capacitor having large capacitance, an electrolytic capacitor is well known. Examples of the electrolytic capacitor are an electrolytic capacitor using an electrolyte (such as an aluminum electrolytic capacitor) and a solid electrolytic
Recently, a conducting polymer prepared by doping a polymer, such as polypyrrole and polythiophene, which is obtained by electrolytically polymerizing a heterocyclic monomer such as pyrrole or thiophene, with BF4xe2x88x92 (borofluoride ions) or C104xe2x88x92 (perchlorate ions) has been proposed as a solid electrolyte usable instead of manganese dioxide (Japanese Laid-Open Patent Publication No. 60-37114/1985).
The conducting polymer obtained by using the aforementioned halogen anions as a dopant, however, can easily degrade a dielectric film. In addition, the conducting property of the conducting polymer can be easily lowered because it is poor in thermal stability and can be easily undoped. The conducting polymer can be easily undoped particularly in fixing an electrode at a high temperature of approximately 200xc2x0 C.
The degradation of the dielectric film and the lowering of the conducting property of the conducting polymer can increase a leakage current in a capacitor, resulting in reducing the capacitance and increasing the impedance.
A conducting polymer obtained by using aryl sulfonic acid ions such as dodecylbenzene sulfonic acid ions and naphthalene sulfonic acid ions, as a dopant has been proposed for overcoming the aforementioned disadvantages (Japanese Laid-Open Patent Publication No. 64-49211/1989).
The conducting polymer obtained by using aryl sulfonic acid ions as a dopant has still rather high electric resistance although the electric resistance is lower than that of the conducting polymer obtained by using BF4xe2x88x92 or C104xe2x88x92 as a dopant. Therefore, even when this conducting polymer is used as a solid electrolyte, it is difficult to obtain a solid electrolytic capacitor exhibiting low impedance in a high frequency region.
The present invention was devised in view of these conventional disadvantages, and a main object of the invention is providing a solid electrolytic capacitor having large capacitance and exhibiting low impedance in a high frequency region.
A solid electrolytic capacitor of the invention (hereinafter referred to as a first capacitor) comprises an anode, a dielectric film formed on the anode by anodic oxidation, a solid electrolytic layer formed on the dielectric film made from a conducting polymer produced by doping a polymer including a heterocyclic monomer unit represented by the following general formula as a repeating unit with alkyl sulfonic acid ions and sulfuric acid ions, and a cathode connected with the solid electrolytic layer. 
Another solid electrolytic capacitor of the invention (hereinafter referred to as a second capacitor) comprises an anode, a dielectric film formed on the anode by anodic oxidation, a solid electrolytic layer formed on the dielectric film made from a conducting polymer obtained by doping a polymer including a heterocyclic monomer unit represented by the following general formula as a repeating unit with alkyl sulfonic acid ions in a ratio of one of the alkyl sulfonic acid ions per 2 through 5 heterocyclic monomer units represented by the general formula, and a cathode connected with the solid electrolytic layer. In this specification, the first capacitor and the second capacitor may be generically referred to as the capacitors of the invention. 
wherein R1 and R2 independently indicate an alkyl group or H, and X is S or NR3 (wherein R3 indicates an alkyl group or H).
The solid electrolytic layer is preferably formed by electrolytic polymerization using, as a solution for electrolytic polymerization, an aqueous solution including a heterocyclic monomer such as pyrrole or thiophene and an ionic dopant such as alkyl sulfonic acid ions, because the dopant can be uniformly doped in the polymer in this manner.
The conducting polymer used in the first capacitor is preferably doped in a ratio of one ion of the dopant per 2 through 5 heterocyclic monomer units each represented by the above-described formula. The molar ratio between sulfuric acid ions and alkyl sulfonic acid ions used for doping in electrolytic polymerization is not specified. It is, however, confirmed through experiments that the maximum molar ratio is 10:1. The sulfuric acid ions cannot be doped exceeding this molar ratio. ions such as dodecylbenzene sulfonic acid ions and naphthalene sulfonic acid ions, as a dopant has been proposed for overcoming the aforementioned disadvantages (Japanese Laid-Open Patent Publication No. 64-49211/1989).
The conducting polymer obtained by using aryl sulfonic acid ions as a dopant has still rather high electric resistance although the electric resistance is lower than that of the conducting polymer obtained by using BF4xe2x88x92 and C104xe2x88x92 as a dopant. Therefore, even when this conducting polymer is used as a solid electrolyte, it is difficult to obtain a solid electrolytic capacitor exhibiting low impedance in a high frequency region.
The present invention was devised in view of these conventional disadvantages, and a main object of the invention is providing a solid electrolytic capacitor having large capacitance and exhibiting low impedance in a high frequency region.
A solid electrolytic capacitor of the invention (hereinafter referred to as a first capacitor) comprises an anode, a dielectric film formed on the anode by anodic oxidation, a solid electrolytic layer formed on the dielectric film made from a conducting polymer produced by doping a polymer including a heterocyclic monomer unit represented by the following general formula as a repeating unit with alkyl sulfonic acid ions and sulfuric acid ions, and a cathode connected with the solid electrolytic layer. Another solid electrolytic capacitor of the invention (hereinafter referred to as a second capacitor) comprises an anode, a dielectric film formed on the anode by anodic oxidation, a solid electrolytic layer formed on the dielectric film made from a conducting polymer obtained by doping a polymer including a heterocyclic monomer unit represented by the following general formula as a repeating unit with alkyl sulfonic acid ions, and a cathode connected with the solid electrolytic layer. In this specification, the first capacitor and the second capacitor may be generically referred to as the capacitors of the invention. 
wherein R1 and R2 independently indicate an alkyl group or H, and X is S or NR3 (wherein R3 indicates an alkyl group or H).
The solid electrolytic layer is preferably formed by electrolytic polymerization using, as a solution for electrolytic polymerization, an aqueous solution including a heterocyclic monomer such as pyrrole or thiophene and an ionic dopant such as alkyl sulfonic acid ions, because the dopant can be uniformly doped in the polymer in this manner.
The conducting polymer is preferably doped in a ratio of one ion of the dopant per 2 through 5 heterocyclic monomer units each represented by the above-described formula. The molar ratio between sulfuric acid ions and alkyl sulfonic acid ions used for doping in electrolytic polymerization is not specified. It is, however, confirmed through experiments that the maximum molar ratio is 10:1. The sulfuric acid ions cannot be doped exceeding this molar ratio.
It is preferable to use aluminum or tantalum as an anode material because a dielectric obtained by anodic oxidation provides a high dielectric constant and a high electrical insulating property. Since the dielectric film is formed by the anodic oxidation, when the anode is made from aluminum or tantalum, the dielectric film is formed from an oxide of aluminum or tantalum.
The dopants used in the fabrication of the capacitors of the invention are not halogen anions such as BF4xe2x88x92 and C104xe2x88x92 but are non-halogen anions, and hence, the dielectric film is difficult to degrade. Also, the alkyl sulfonic acid ions exhibit higher thermal stability and are more difficult to undope as compared with BF4xe2x88x92 and C104xe2x88x92 even when they are exposed to a high temperature, for instance, in fixing an electrode. Therefore, the electric conductivity of the conducting polymer is not easily lowered. For these reasons, in the capacitors of the invention the leakage current is small and the deterioration of properties is small even when they are exposed to a high temperature. Moreover, since the electric resistance of the conducting polymer doped with alkyl sulfonic acid ions is lower than that of the conducting polymer doped with aryl sulfonic acid ions, the capacitors of the invention exhibit lower impedance and better frequency characteristic as compared with the solid electrolytic capacitor disclosed in Japanese Laid-Open Patent Publication No. 64-49211/1989.